Elias Gottlob Haussmann, portrait of Johann Sebastian Bach, 1746. Via Wikimedia Commons (public domain).
The fugue is the piece that makes people run, and the reason is always the same: you try to listen to it the way you listen to a song, that is, by following the main melody, and after twenty seconds you no longer know where the main melody has gone. It is not a problem of the ear. It is that in a fugue the main melody does not exist in the singular; there are three or four at once, and listening has to be organised another way.
The mechanism is simple and worth knowing beforehand, because once you know it there is no going back. At the start there is a single voice, and that voice states a short theme, five to fifteen seconds long. That theme is called the subject and it is the only thing to learn by heart: it is brief and it returns throughout the piece. Then a second voice enters with the very same theme, but shifted up a fifth or down a fourth, while the first goes on doing something else above or below. Then a third. Then a fourth. The exposition is over when every voice has stated the theme once.
From there on there is only one game: the theme disappears, reappears in a different key, disappears again. The stretches where it is absent are called episodes and serve to move the music elsewhere. Listening to a fugue therefore means doing one thing only: waiting for the subject to return, recognising it when it arrives, and noticing in which voice it has come. It is a hunt, not a contemplation.
One practical piece of advice that changes everything: on a first listening, never follow the highest voice. The ear goes there on its own, and it is the best way to get lost. Follow the bass. The low voice in a fugue is the one that moves least and holds the structure, and when the subject enters in the bass you feel it like a blow, because there is nothing else underneath. Whoever learns to stay on the bass understands half of Western music, not just fugues.
Then there are the tricks composers use when the piece is halfway through and the stakes need raising. The stretto: the voices enter with the theme one on top of another without waiting for the previous one to finish, and the effect is of crowding, of people talking over someone who has not finished the sentence. Augmentation: the theme returns with its values doubled, that is, at half speed, usually in the bass, and becomes monumental. Diminution, which is the opposite. And inversion, that is, the theme turned upside down: where it rose, it now falls. Four effects you can recognise without knowing any theory, you only need to know they exist.
Where to begin, if you want to run the experiment tonight. Not with the Well-Tempered Clavier, which has beautiful fugues but often in four voices and tightly woven. Better to start from the Art of Fugue, the first number, which is the barest fugue Bach ever wrote: a very simple theme in D minor, four voices, everything exposed, and built on purpose so that you can see how it works. Bach left it unfinished and the volume came out posthumously in 1751, with the final contrapunctus breaking off mid-phrase, and at that point, in the score, his son Carl Philipp Emanuel wrote by hand that here the author had died.
Third listening, and it is there to show that the fugue is not a relic. In the finale of Mozart’s Symphony No. 41 and in half the chase soundtracks of the last forty years the procedure is the same: staggered entries, accumulation, stretti. The cinema understood before the concert hall that this form serves to build tension, because the ear perceives the accumulation of voices as a rise in pressure even when it does not know why.
One last thing. If on a first listening you get lost anyway, that is fine, you get lost. The fugue is the only musical form that is learned by listening to the same piece five times instead of five different ones. By the third time the theme starts to jump out on its own, and then the strange thing happens: you can no longer not hear it.